Vital functions can be measured telemetrically using a non-invasive measuring device. An example of such a measuring device is a system which measures human heart rate and usually comprises several functional units, such as a transmitter unit, receiver unit and data transmission unit. Each functional unit usually comprises a battery which functions as the unit's power source. The transmitter unit means a unit which is provided with electrodes and is held against the human body, particularly against the chest. This unit is often implemented as a transmitter belt attached around the body. The receiver unit refers to a watch-like unit which is worn on the wrist, for example, and which telemetrically interacts with the transmitter unit on the basis of inductive activity. The data transmission unit, which communicates telemetrically with the receiver unit, is used for transmitting data collected in the receiver unit to a computer, for example. The computer can also be used for controlling both the transmitter and the receiver units via the data transmission unit.
Measurement of the heart rate is based on monitoring the function of the heart. When the heart muscle contracts, it causes a series of electric impulses which can be measured in the body. The measurement and analysis of this signal is known as electrocardiography (EKG). The signal itself is called an EKG signal. Different phases of the heart cycle can be discerned in the EKG signal. These are called P, Q, R, S, T and U waves. Sensors measuring height and water depth, for example, can be connected to the measuring system.
The power sources of the different functional units, which are typically batteries, are changed after the voltage level has decreased too much. This complicates the use of the device since even though the charge of a battery decreases in a known way, the fact that the electric energy of the power source has run out usually surprises the user e.g. in the middle of exercise.
The unit that receives the heart rate usually comprises a piezoelectric sound signalling device, electroluminescent light source and at least a receiver for the heart rate. For the function of each device the prior art solutions comprise a separate induction coil. Since the receiver unit, in particular, is typically a device similar to a wrist watch, the space needed for several coils constitutes a major problem. The use of several coils also raises the price of the device.